Charged walls in implanted garnets

The domain structure around non-implanted discs in implanted garnets has been investigated as a function of the strength of in-plane fields, H_| is in a hard direction, the observed domain structures can be explained by the theory of Shir and Lin [14]. When H_| is in easy direction or at 30° to it, charged walls are still present for values of H_| for which the theory predicts saturation of the implanted layer. This results is attributed to demagnetizing effects. The dependence of the length of charged walls on the diameter of non-implanted discs and on H_| has been studied.     

Crystal field interaction in erbium garnets

A crystal field analysis has been performed for erbium aluminum garnet (ErAG) and erbium gallium garnet (ErGG). Good agreement between the measured and calculated energy levels was obtained.     

Magnetization measurements on some rare-earth iron garnets

The magnetic properties of Fe-based singlecrystal garnets have been studied. A magnetic field up to 55 kOe in the temperature range from 4.5 to 300 K has been used. The compensation temperature (T _comp) of the studied garnets has been determined. The compensation temperature increases with the increase of the rare-earth ions (Ho³⁺ or Gd³⁺) substituting Y³⁺ ions in thec sublattice of R₃Fe₅O₁₂. AboveT _comp, the magnetization was found to be linearly field dependent. The results are in good agreement with magnetization measurements performed on polycrystaline samples, and with calculations based on the crystal field parameters of the isostructural Holmium Gallium Garnet (HoGG).     

Cation distribution in Eu-Sc-Fe garnets

The cation distribution has been studied with ⁵⁷Fe Mössbauer spectroscopy in the garnet system Eu_3?ySc_2+yFe₃O₁₂ with y = 0.0, 0.2 and 0.5. It is shown that the previously proposed cation distribution is not correct. The problem of possible impurities in the investigated system is discussed in detail. Several possible cation distributions are considered compatible with Mössbauer data. Mössbauer results combined with the composition dependence of lattice constants show that the tetrahedral sites are accessible to Sc³⁺ ions. The system studied is a second example of a garnet structure in which Sc³⁺ ions are found at the tetrahedral sites. A small fraction of Sc³⁺ ions for the samples y = 0.2 and 0.5 is also found at the dodecahedral sites.     

Magnetic structure of ion-implanted bubble garnets

The surface magnetic structure of bubble garnet films implanted with 80 keV Ne⁺ ions has been investigated by conversion-electron Mössbauer spectroscopy in conjunction with backscattered X-ray Mössbauer spectroscopy. For lower doses (～1–3 × 10¹⁴Ne⁺cm^-2) a ferrimagnetic component with in-plane magnetization coexists with a smaller paramagnetic phase in the implanted layer; for a dose of 5 × 10¹⁴Ne⁺cm^-2 only a paramagnetic phase is observed.     

Site preference of iron,gallium and aluminum in quasibinary mixed garnets

The temperature dependence of the equilibrium distribution of Ga resp. Al substitutions over the a-and d-sites in single crystal yttrium iron garnet has been determined from magnetization measurements for substitution levels 0 < z < 1.5 and equilibrium temperatures between 773 and 1773 K. Including literature data derived by different experimental methods for as grown samples of various substituted garnet systems with 0 < z < 5 it is shown that the cation distribution observed on single crystals and polycrystalline samples can be well described by a thermodynamic equilibrium distribution model with two stabilizing energies as fitting parameters. The model includes the influence of the level of substitution and of the actual distribution on the effective site stabilizing energy. By a least error fit to the experimental data the two fitting parameters are determined in terms of the site stabilizing energies for the end member garnets with substitution levels z → 0 and z → 5, respectively: 0.272eV and 0.230eV for Ga substituted yttrium iron garnet: 0.178eV and 0.132eV for Al substituted yttrium iron garnet; 0.26 eV and 0.11 eV for Ga substituted yttrium aluminum garnet; 0.20 eV and 0.15eV for Ga substituted europium iron garnet.     

High magnetic field properties of mixed terbium-yttrium ferrite garnets

Magnetization measurements in d.c. magnetic fields up to 180 kOe have been performed on polycrystalline specimens and on single crystals of mixed terbium-yttrium iron garnets: Tb _x Y_3–x Fe₅O₁₂ in the 4.2–300 K temperature range. On polycrystals (x=2.5; 2; 1; 0.37) the concentration dependence of both compensation point and spontaneous magnetization has been determined. In the single crystals (x=2; 1; 0.37) a strong magnetic anisotropy has been observed with a change of the easy direction of magnetization from [111] to [100] whenx decreases; abrupt field-induced transitions have been observed at low temperatures when the external field is applied along the [100] direction.     

Kinetics of cation site exchange in mixed garnets

The redistribution rate of cations between the a- and d -sites in quasibinary mixed yttrium—iron gallium and—iron aluminum garnet single crystals has been investigated for temperatures between 773 and 1523 K. The frozen-in cation distributions of quenched and of slow cooled samples were derived from magnetization measurements and are described by a phenomenological kinetic equation. The activation energies derived for the thermally activated process range from 1.3 to 4eV and are related to the anion vacancy concentration of the crystals. The zero defect intrinsic a-dbarrier height is 4.5 eV as inferred from the observed linear relation between the frequency factor and the activation energy of the Arrhenius equation. This empirical relation, known as the compensation law, has been further applied to interpret reported intra-sublattice site exchange effects of cations on the c-sites and cation self-diffusion data of YIG. The inferred c-c barrier height of 5.5 eV agrees within the error limits of ± 0.1eV with the enthalpy of formation, 5.4 eV, and the enthalpy of solution, 5.3 eV, derived with equilibrium models based on the ionic fluid (Temkin) model and using experimental results of the kinetics, the cation distribution and the solubility of garnets. Apparently the c-c bonds form the strongest binding scaffold of the garnet structure, which is manifested by a relation to the melting point. While premelting around the anion vacancies assisted by their thermal fluctuations reduces the effective barrier height for cation migration, the apparently uneffected melting point of the compound itself is reflected by the compensation law.     

Dissolution forms of rare-earth gallium-aluminium garnets

Acta Physica Hungarica - Single crystal spheres of rare-earth gallium-aluminium garnet crystals (R3Al5?x Ga x O12, whereR=rare-earths and 0≤x≤5) have been dissolved in H3PO4 and...     

Changes in trap parameters in various mixed oxide garnets

Shallow and deep electronic traps in various mixed oxide garnet ceramics (Y,Lu,Gd)₃(Al,Ga)₅O₁₂ have been studied by thermally stimulated luminescence spectroscopy in the 80–550 K temperature range. It is shown that the substitution of Al ions by Ga and Y by Gd or Lu in YAG:Ce affects the properties of the traps. It is established that the studied ceramics contain residual impurities of chromium and ytterbium. On the base of the obtained and literature data, a table of the Cr-related trap position relative to the bottom of the conduction band in RE₃(Ga_x,Al_5-x)₅O₁₂:Ce garnets (RE = Lu, Y, Gd and combinations thereof) has been constructed.     

Electric field gradient calculations in rare-earth iron garnets

The electric field gradient (EFG) tensor at the three cation sites in RE₃Fe₅O₁₂ garnets (RE = Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu) has been calculated using a monopole-point-dipole model. The monopole contribution to the EFG tensor can explain the signs and the changes with the RE radius of the experimental quadrupole coupling constants at the octahedral and the tetrahedral sites. The monopole and dipole contributions together cannot account for the magnitude of the experimental tensors at these sites for any value of oxygen dipole polarizability α. The dipole contribution is important in garnets even for very small values of α. Presently available calculations of the overlap contribution to the EFG tensor combined with the lattice contribution also do not agree with the experimental data for these two sites. A possible cause of the disagreement is given. It is suggested that accurate experimental data of the quadrupole coupling constants at the dodecahedral sites in a RE₃Fe₅O₁₂ series, especially in Gd₃Fe₅O₁₂, could resolve the importance of various contributions at all three sites.     

Spectroscopic investigation of some rare earth iron garnets

Optical absorption spectra of Dysprosiumirongarnet (DyIG), Holmiumirongarnet (HoIG) and Erbiumirongarnet (ErlG) have been investigated in the near infrared region at temperatures of 4.2°K and 62°K. Exchange field splittings of the crystal field levels have been observed. — For the three garnets the excited levels of the groundterm up to 110 cm^?1 could be determined. The exchange fields acting on the rare earth ions have been deduced as (305±50)·10³ Oe for DyIG and (284±30)·10³ Oe for ErIG.     

Anti-ferromagnetic ordering and crystal electric field in Ce:GSAG and Ce:LLGG garnets

The magnetic susceptibility of Ce:Gd₃Sc₃Al₃O₁₂ (Ce:GSAG) and Ce:La₃Lu₂Ga₃O₁₂ (Ce:LLGG) has been studied in the temperature range from 15 to 293 K. Both the Ce-doped garnets Ce:GSAG and Ce:LLGG show anti-ferromagnetic ordering at 18 and 27 K, respectively. Crystal electric field split energy levels have been used to calculate the magnetic susceptibility following Van Vleck's model. The calculated susceptibility values reasonably agree with the experimental susceptibility data of Ce:LLGG but differ for that of Ce:GSAG system. Contribution from long-range ordering between Ce and Gd ions for anomaly in Ce:GSAG system is speculated.     

Luminescence of excitons in single-crystal garnets

Comparative studies of the luminescence of Y₃Al₅O₁₂:Ce and Lu₃Al₅O₁₂:Ce single-crystal films and their volume analogues—Y₃Al₅O₁₂ and Y₃Al₅O₁₂:Ce single crystals, excited by synchrotron radiation with energy E=120–150 eV, have been performed. The films were grown from melt-solution by liquid-phase epitaxy and the crystals were grown from melt. The single-crystal films and single crystals studied are characterized by different degrees of structural order, in particular, different concentrations of substitutional defects of the Y _Al ³⁺ and LU _Al ³⁺ types. It was ascertained that the bands at 260 and 250 nm in the intrinsic luminescence spectra of Y₃Al₅O₁₂:Ce and Lu₃Al₅O₁₂:Ce single-crystal films and single crystals are due to the emission of self-trapped excitons. The luminescence band with λ_max=300 nm and τ=0.36 μs, which is present in the luminescence spectrum of single crystals and absent in the spectra of single-crystal films, is due to the recombination of electrons with holes localized at Y _Al ³⁺ centers. It is shown that an efficient energy transfer by excitons to activator ions occurs in Y₃Al₅O₁₂ and Lu₃Al₅O₂ single-crystal films doped with Ce³⁺ ions.     

Evidence for non-cubic magnetostriction in epitaxial bubble garnets

Epitaxial bubble garnet films grown on non-magnetic garnet substrates exhibit a dominant growth or stress induced uniaxial anisotropy, which is responsible for the stripe and bubble domain structures, and the intrinsic cubic magnetocrystalline anisotropy which can affect bubble device performance. The anisotropy constants have been deduced from measurements of stripe domain nucleation in the garnet films. We extend this measurement technique and its interpretation so that it also yields values of the magnetoelastic interactions.The measurement is based on observing the details of the topography of the nucleating domain structure, specifically the orientation of the nucleating stripe domains as a function of the orientation and magnitude of the applied magnetic field.The interpretation is based on a micromagnetic analysis of the conditions for homogeneous second order stripe domain nucleation. The contributions to the phenomena of the cubic anisotropy and of the magnetostriction are included in the analysis as perturbations.The theory produces predictions which are compatible with qualitative earlier experiments reported in the literature. It provides a satisfactory quantitative account of systematic new observations we have made on a GdTmY bubble garnet film with the specific objective of measuring magnetostriction.Analysis of the experimental data yields strong evidence for a non-cubic component of the magnetostriction possibly associated with the same growth-kinetic mechanism that gives rise to the non-cubic anisotropy. The sign and magnitude of the macroscopic non-cubic magnetoelastic constant is estimated from the experimental results.     

Magnetic phase transitions in garnets

The phenomenological theory broadly applicable to magnetic transitions in ferrimagnetic garnets is discussed briefly. The experimental techniques, particularly nuclear magnetic resonance and Mössbauer effect spectroscopy, are then reviewed. Finally, there is a review of the results on specific garnets which undergo such transitions. Some remaining problems are pointed out.     

Magnetooptics of magnetic-dipole transitions in the rare-earth paramagnetic garnets

The Faraday effect (FE) of the rare-earth (RE) paramagnetic garnet crystals associated with magnetic-dipole (MD) transitions has been investigated theoretically over a wide temperature range 80 to 300 K. Over this temperature range, the ??gyromagnetic?? Verdet V _gm ^(m) of the Eu₃Ga₅O₁₂ (EuGG) is not a linear function of the magnetic susceptibility ?? measured along the [111] axis. We find that the nontrivial character of the temperature dependence of the MD Verdet constant V _gm ^(m) is associated with the presence of a contribution to the Faraday rotation of the Van Vleck ??mixing?? by an external magnetic field of the Eu³⁺ electronic states. The optical absorption and magnetic circular dichroism (MCD) of Eu³⁺ ion in EuGG at 85 K for the MD transition ⁷ F ₀ ?? ⁵ D ₁ were measured along the [111] axis. We show that the appearance of the MCD feature observed for the absorption line near 526.6 nm for the ⁷ F ₀ ?? ⁵ D ₁ MD absorption band may be well-explained by the contribution of Van Vleck ??mixing?? between the Stark singlet states from the ⁷ F ₀ and ⁷ F ₁ multiplet manifolds mixed by the external magnetic field H.     

Preparation and optical properties of single crystal thin films of bismuth substituted iron garnets for magneto-optic applications

Single crystal thin films of Bi substituted iron garnets have been grown by the liquid phase epitaxy technique. The growth conditions and compositions are presented together with the segregation coefficients of Bi and Pb as a function of growth temperature. The Faraday rotation and optical absorption have been measured for wavelengths between 0.7 and 0.46 μm. The superior magneto-optic properties (Faraday rotations of up to ten times larger than that observed for Y₃Fe₅O₁₂) make these epitaxial layers attractive for magneto-optic devices. The influence of Pb incorporation on the optical properties of the films is discussed. Compositions of Bi and Ga(Al) substituted Gd₃Fe₅O₁₂, which exhibit a magnetic compensation temperature near room temperature and high Faraday rotation, have also been prepared. The magnetic switching behaviour as a function of temperature has been determined. It shows that these films are especially suited for thermomagnetic recording using an argon ion laser at a wavelength of 5145 Å.     

Luminescence of germanate garnets with terbium on dodecahedral and octahedral lattice sites

{Ca(Tb)}₃Ga₂Ge₃O₁₂ and Sr₃[Y(Tb)]₂Ge₃O₁₂ garnets provide a cubic lattice with trivalent terbium, a luminescent rare earth, in dodecahedral (8 oxygen neighbors) and in octahedral (6 oxygen neighbors) sites, respectively. In the materials examined, Tb³⁺ in octahedral sites has a narrower fluorescence emission spectrum, higher luminescence efficiency and a longer lifetime than in dodecahedral sites. The luminescence properties and the unit cell size of the Ca₃Y₂Ge₃O₁₂ garnet, where Tb³⁺ is expected to be located on octahedral sites, are between those of the CaGa and SrY garnets. All these garnets are phosphorescent. On the basis of the variation with temperature of the phosphorescence decay law, it is proposed that the radiative electron-hole recombination process proceeds via a tunneling mechanism at temperatures below the thermal glow peaks, and via a free carrier diffusion mechanism at temperatures above the thermal glow peaks. All three Tb³⁺-doped germanate garnets have sufficiently high efficiency and persistence under electron beam excitation to be suitable for practical phosphor applications. The strontium yttrium germanate-Tb material has a cathodoluminescence decay time nearly three times that of the commercial P53 garnet phosphor.     

Exchange interactions in rare-earth iron garnets

Exchange parameters previously reported for YbIG, ErIG and EuIG are compared. One of the three possible sets of parameters found for ErIG is shown to agree fairly well with the EuIG parameters and can also be fitted using an exchange potential. Tentative exchange potential parameters associated with the two types of superexchange pathways which couple the rare-earth ion to neighboring iron ions are obtained. A new method for calculating the YbIG exchange parameters is described and the parameters are compared with those reported previously.